This invention relates to methods for in situ immobilization of metals in water and earthen boundaries bordering the water as well as to immobilization treatment of metals in water having varying density zones to access and treat all regions within the water and at the water-earthen boundaries.
Waste stacks are generated by many types of industrial processes, often as a result of the extraction of valuable materials. The waste stacks are frequently piles of economically invaluable material left over from the industrial processes. For instance, power plants often generate waste stacks of ash. The ash is left over when energy is extracted from fuel by burning. Mining processes also often generate waste stacks. The waste stacks contain minerals left over after a valuable metal or mineral is extracted from the mined earth materials. For example, phosphorus mines often result in waste stacks containing predominantly gypsum as a processed waste. The waste stack gypsum is a relatively invaluable mineral left over after phosphorus is removed from the mined materials.
In many instances, waste stacks are formed as follows. First, the residual or waste material is combined with water to form a waste slurry. This waste slurry is then flowed to a settling pond where the solids contained in the waste slurry settle out. Water evaporates or permeates from the settling pond. Over time, the settled solids leave behind a stack of waste material. Some water is retained in the settled waste material which makes up the waste stack. This process of deposition settling and evaporation is repeated until the resulting waste stack is too large for the process to economically continue, or is terminated for other reasons. If needed, a new waste stack is started and grows in a similar fashion. FIG. 1 shows a mine 12 which has been in operation for a significant period of time and is surrounded by a number of waste stacks 14. The individual waste stacks 14 are often huge, frequently comprising millions of cubic yards. The amount of material currently stored in waste stacks is enormous, and it continues to increase as mining and other industries continue to produce and develop new operations.
A problem associated with waste stacks is toxic metal migration. The actual percentage of water-soluble toxic metals in a given waste stack is usually very small; for example, less than 1.0 percent. Because the waste stacks are often very large, however, the total amount of toxic materials in a waste stack is often large enough to present some risk to surrounding areas and ground water. These risks arise in part due to potential metals migration of liquids from the waste stack. The slurry water may percolate into the soil in addition to evaporating or remaining in the waste stack.
Toxic metals potentially found in waste stacks include but are not limited to Pb, Hg, U, Cd, Fe, As, Se, Cu, Cr, Ni, Zn, Co, Mn, and Ag. Over time such metals can leach out of the waste stacks and into ground water. Thus, it is desirable to keep the metals within or near the waste stacks to minimize the danger posed by such metals.
Keeping the metals within or near the stack is often difficult, especially since the metals may be present in water-soluble forms. Such water-soluble forms can migrate as metal solutes whenever water moves through the stacks. Since the stacks are frequently exposed to water, either in the form of rain or in the form of wastewater deposited on the stacks, water-soluble metals or metal compounds present in the stacks are exposed to conditions that may encourage their migration. In some situations, metals have already begun migrating out of existing waste stacks and into a boundary zone or layer below the waste stacks. Thus, it is desirable to have a method which will not only inhibit further migration of metals from the waste stacks, but which will also inhibit the migration of metals that are in a boundary layer beneath the waste stacks.
One method for containing metals within a waste stack has been to treat the waste stack with microbes that are capable of producing microbial sulfides. The microbial sulfides are sulfide byproducts of microbial activity in waste stack affected zones. The microbial sulfides react with metal ions or metal containing compounds contained in the waste stack affected zones to form metal sulfides. The metal ions or metal containing compounds contained in the waste stack affected zones become relatively insoluble during this treatment and are inhibited from migrating within or from the waste stack affected zone. This method was first disclosed in U.S. Pat. No. 5,632,715, which is incorporated herein by reference. This method has been applied successfully in treating waste stacks and inhibiting migration of metals within boundary areas within such waste stacks.
With the excavation of mining materials during the mining process, a void is typically created that is often filled with water. These large water-filled zones, typically known as pit lakes 15, contain many of the same type of contaminants as are found in waste stacks, especially when the lake is adjacent a waste stack from the mining operation. Additionally, the pit lakes have soil boundaries along the surface as well as extending down to the bottom of the lake. Metal migration continues to occur within the soil boundaries between the water and the soil. Further still, the water from the pit lakes can seep into adjacent water tables, which can result in the contamination of water systems in populated areas.
One prior art method of treating such bodies of water has been to pump the water from the lake source to a process treatment plant and then return the treated water to the pit lake. Another method in the prior art for treating such bodies of water has included taking the process treatment plant to the body of water and placing it on a boat that travels across the surface of the pit lake to treat the water at the surface level and return the treated water back to the surface.
There are several problems that exist in either solution. Firstly, both treatment solutions are expensive to conduct, as the cost of pumping the water alone can be extreme. Secondly, mixing treated water with contaminated water only causes the contaminated clean water to be re-contaminated, or require there be a secondary storage facility, which is not always available or suffers from the same soil contamination of the first pit lake. Thirdly, the plant operators must be on the water in the process on the lake method, which potentially exposes the operators to the contaminated water unnecessarily. Fourthly, these treatment solutions are at times unable to reach the depths of these pit lakes, which can have depths ranging from 50 feet to as great as 3,000 feet. Again, water pumping becomes expensive for deep pit lakes. Fifthly, certain water-filled workings are completely subterranean and are virtually impossible to access directly or the water is so deep that pumping the water from the subterranean cavity to the surface for treatment becomes cost prohibitive.
Not only must the water be treated in such conditions, but so to must the soil boundary within either the subterranean pit lake or the open body pit lake also be treated during the treatment process. The prior art methods of removing the water for treatment at a separate location, or merely treating the water on the surface of the pit lake fails to treat to treat the soil boundary of the lake simultaneously with treating the water.
Accordingly, there is a need within the industry to be able to treat contaminated water sources, such as pit lakes and subterranean mine cavities filled with water, in an economical and environmentally sound way that also includes treating the soil boundaries adjacent the water.